1. Field of the Invention
The present invention relates to a magnetic recording medium. More particularly, the present invention relates to a magnetic recording medium having excellent running stability and durability, and being less likely to bring about clogging and dropouts.
2. Description of the Prior Art
Recently, magnetic recording media have been widely used in the fields of audio and video recording, computers, magnetic discs, etc. Accordingly, the amount of information to be recorded on magnetic recording media increases year after year, and there is an increasing demand for the improvement of the recording density of magnetic recording media.
A non-binder type magnetic recording medium, i.e. a magnetic recording medium comprising a ferromagnetic thin film, has advantages over a coating type magnetic recording medium in that it can readily be made thin and it has great saturation magnetization. Nevertheless, it has some problems in respect of corrosion, impact strength and frictional strength, and is susceptible to abrasion or destruction due to its engagement with the magnetic head in high speed relative movement during the operation such as recording, playing back or erasing of magnetic signals. Namely, the magnetic recording medium with a ferromagnetic thin film formed by electroplating, electroless plating, sputtering, vacuum vapor deposition, ion plating, etc., contains no binder, and accordingly, the magnetic recording layer is likely to be abraded or destructed by friction during the sliding engagement with the magnetic head.
Further, the magnetic recording medium comprising a ferromagnetic thin film is likely to be corroded from its surface, and if the corrosion progresses, the practically desired properties such as head touch or abrasion resistance deteriorate, and the electromagnetic conversion characteristics will be adversely affected.
In order to overcome this difficulty, it has been proposed to apply a lubricant to the surface of the magnetic thin metal film (Japanese Examined Patent Publication No. 25246/1964). However, in such a method, the lubricating effect does not last as the lubricant is likely to be wiped off by e.g. the magnetic head. Further, such effects as rust-proofing effects or durability can not of course be expected by this method.
Further, as a means to provide a lubricating effect continuously on the magnetic recording layer, it has been proposed to provide a lubricating layer (i.e. a back coating layer) formed on the opposite side of the magnetic recording layer and comprising a liquid or semisolid lubricant and an organic binder as main components (Japanese Examined Patent Publication No. 29769/1982). In this method, the lubricant exuding from the rear side surface of the magnetic recording layer transfers to the magnetic recording layer when the magnetic medium is wound in the form of a roll, whereby the lubricant can always be supplied to tne surface of the magnetic recording layer, and the durability (resistance against scratching or abrasion) or the dynamic frictional coefficient of the magnetic recording layer are said to be improved. However, in such a method wherein a lubricant is incorporated in the back coating layer without providing a top coating layer on the ferromagnetic thin film, the level of friction between the magnetic thin film and the magnetic head is still high, thus leading to a running failure, and the corrosion resistance or the rust-proofing effect is not adequate.
Also in the case of a coating type magnetic recording layer, it is known to provide a top coating layer in order to protect the magnetic layer. However, such a conventional top coating layer has a drawback that it is likely to be abraded.
The present inventors have conducted extensive researches to overcome the above difficulties, and have found and previously proposed that it is possible to obtain a magnetic recording medium having improved running property and durability, by providing a top coating layer comprising specific components on the surface of a magnetic recording layer (Japanese Patent Application No. 15258/1984 and No. 15259/1984). From a further research, the present inventors have now found that the above-mentioned conventional top coating layer is likely to be abraded, because it contains no fine particulate pigment, and thus has no head cleaning function. Further, they have found that it is possible to overcome the above drawback and obtain an excellent magnetic recording medium having stability with a minimum possibility of clogging, level down and dropout when the top coating layer contains a fine particulate pigment at its surface and the surface roughness (R20) of the top coating layer is less than 400 .ANG.. The present invention has been accomplished on the basis of these discoveries.